The present invention relates to switches, and more particularly, to switches that terminate input transmission lines in their characteristic impedance.
Frequently, in television systems it is desired to couple a selected one of a plurality of input sources to an output. The input sources are coupled to the switch by a transmission line, which also has other users coupled to the line. Typically the nonselected lines are simply left open by the switch. This causes reflections on the nonselected lines, thereby resulting in ghosts in the displays of the other users.
Another problem arises from the fact that typical prior art switches are constructed using a pi-configuration of series and shunt connected diodes. When the switch is off (not passing signals), the series diodes must be reversed biased, while the shunt diodes are forward biased, and therefore conducting current. To achieve a sufficient amount of attentuation (about 62 db) between an input port and the output port when in the off-state, the back-biased series connected diodes must have a low reverse capacitance. To achieve this requires a relatively high reverse biasing voltage due to said series connection, and since the shunt diodes are conducting current, a relatively large power is used.
It is therefore desirable to provide a switch that terminates non-selected input sources in their characteristic impedance and uses relatively low switching power.